The easiest way to convert men's sizes to women's is to take accurate measurements of the bust, waist and hips. A general rule of thumb is that a man should go three inches larger in the waist. If a man's waist is 36 inches, buying a dress that would fit a woman who has a 39 to 40 inch waist should fit well.
It's important to take proper measurements because general sizing can vary based on designer and store. While a size 14 may fit from one designer, a man may need a size 12 or 16 from another designer for a similar dress.